This invention relates in general to compositions of matter and, more particularly, to novel copolymer compositions and products, including dental composites, made from such copolymer compositions.
The shrinkage during polymerization of many types of monomers makes those monomers generally unsuited for use in numerous applications, including as strain-free composites, high-strength adhesives, and precision castings. As an example, when such monomers are used in composites which include high-strength fibers, the polymeric matrix is subject to failure when the polymer shrinks and pulls away from the fibers. Failure of the composite can also occur when the matrix ruptures as a result of voids or microcracks which form in the matrix during polymerization shrinkage.
Polymeric matrices commonly employed in dental composites are based on 2,2"-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)]phenyl propane (BisGMA). A significant problem associated with the use of this monomer in dental applications is the shrinkage which occurs as the monomer is polymerized. The BisGMA monomer itself typically experiences a shrinkage of approximately 5% and, when a low viscosity reactive diluent is combined with the monomer, the shrinkage may average as much as 7.9%. The adverse effects of such shrinkage are believed to include increased postoperative sensitivity, the formation of marginal gaps between the dental restoration and the cavity wall, cracking of the restoration, and microleakage and potential failure of the restoration.
The discovery that spiroorthocarbonates undergo reduced polymerization contraction has led to the suggestion of their use in reinforced composites, including as dental composites. Spiroorthocarbonates are esters of orthocarboxylic acid and have four oxygen atoms bonded to a single carbon atom, with the carbon atom being common to two ring systems. The expansion of the spiroorthocarbonates on polymerization is attributed to a double spiro-cyclic ring opening of the spiroorthocarbonates, resulting in the breaking of two covalent bonds to form one new bond.
Initial attempts to form a homogeneous polymer matrix from certain spiroorthocarbonates and BisGMA resin mixtures proved unsuccessful because of the incomplete polymerization of the spiroorthocarbonates. Thompson et al., J. Dental Research 58:1522-1532 (1979). More recent studies demonstrated that homogeneous mixtures of other spiroorthocarbonates and BisGMA could be obtained. Stansbury, J. Dental Research 70:527; Abstract No. 2088 (1991). However, the presence of a vinyl functionality in these spiroorthocarbonate monomers, in combination with the unsaturation of the BisGMA monomers, resulted in a polymerization shrinkage of 2.4%, making the polymer unsuited for those application requiring slight polymer expansion or minimal shrinkage.
The photocationic-initiated expansion polymerization of alicyclic spiroorthocarbonate monomers and the potential use of the resulting polymers as dental composites have been previously reported by the present inventors, with others. Byerley et al., Dent. Mater. 8:345-350 (1992). The specific spiroorthocarbonates identified by Byerley et al. include cis/cis, cis/trans, and trans/trans configurational isomers of 2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro-[5,5]undecane of the following formula (I): ##STR1## These spiroorthocarbonates were determined to undergo an expansion of 3.5% during homopolymerization and demonstrated acceptable cytotoxicity and genotoxicity properties, making them promising candidates as composite resin matrix materials.
The present inventors, with others, have also previously reported on the preparation of a copolymer of an alicyclic spiroorthocarbonate and an unidentified monofunctional epoxide, with the observation that there were no indications of the formation of small ring compounds as polymerization by-products. Byerley et al., J. Dental Research 69:263; Abstract No. 1233 (1990). The copolymerization of trans/trans-2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxspiro-[5,5]undecane and commercially available multifunctional epoxides was also disclosed in a paper presented by Byerley et al. However, no physical or mechanical properties, including percentage shrinkage, of the copolymer compositions were disclosed.
The combination of other spiroorthocarbonates with epoxy resins have produced copolymer composite matrices exhibiting decreased water permeation, increased toughness, and significantly decreased polymerization shrinkage. In one example, an expansion of 1.6% was observed when 24% of a dinorbornene spiroorthocarbonate was copolymerized with a diglycidyl ether of bisphenol A. Piggott et al., 31st International SAMPE Symposium 541-550 (1986).
It has also been reported that homopolymerization of an epoxy monomer at ambient temperature would result in very minimal shrinkage. Fish et al., Plastic Technology, 1:28-32 (1961).
Despite the advances resulting from the above-noted investigations of the use of spiroorthocarbonates as composite materials, a need still exists for a spiroorthocarbonate copolymer capable of yielding a hard, non-shrinking matrix resin suitable for formulating dental and other composites.